Wait for the application, not the clock, in the strict QA set

The probe exposes settled() and the first-frame outcome; scripts/qa/ready.mjs
waits for drawn frames with the camera at rest and for a pick at the click
position, with a deadline and its reason in the error. The strict scenarios
use these instead of fixed pauses and loading-screen polling, and the
hidden-switch scenario checks how the loading screen was lifted instead of
how long it took.

Each switch scenario now records a draw ledger per engine and runs a
contract monitor; both write to the scenario's evidence file only, with an
empty allowlist until browser runs show what differs on purpose.

scripts/qa-urban-engine-switch.mjs

@@ -1,3 +1,4 @@
+import { whenSettled } from './qa/ready.mjs';
 import assert from 'node:assert/strict';
 import { fromDegrees } from '../src/geo/cartesian3.js';
 import {
@@ -50,7 +51,7 @@ await strict.run({}, async (b) => {
       .length;
   if (bus) {
     // Outside every feed region the layer waits and reads no feed.
-    await new Promise((resolve) => setTimeout(resolve, 3000));
+    await whenSettled(p);
     const outside = await p.evaluate(() => window.qaUrban.getStats());
     await strict.check(
       p,